Energy storage devices, such as capacitors and batteries, are typically housed in a sealed case. Terminals are located on the exterior of the case, for connecting the energy storage device to an electrical circuit. Over the lifetime of the device, the integrity of the components can decline, which may lead to the generation of gas pressure within the case and eventual failure. In addition to the loss of the component, the failure of the energy storage device can lead to rupture of the case and release of toxic and corrosive materials. Some energy storage devices, for example capacitors for use in HVAC equipment, lighting, AC filtering, line conditioning, power conversion, snubber circuits, DC link, pulse applications and high voltage applications, may be provided with a pressure interrupter, which breaks the connections between the device and the terminals, thereby disconnecting the device. When the pressure interrupter activates, however, the device is disconnected from service, without warning.
Prior to actual failure or activation of a pressure interrupter, the case of the energy storage device may begin to deform outward. Various methods and apparatus for detecting the onset of an outward expansion of the case have been disclosed in the prior art.
U.S. Pat. No. 4,869,197—Gupta et al. disclose a capacitor casing to which a frangible strip of material is attached. When the case deforms in response to a pressure buildup, the strip of material fractures, providing visual indication of the event.
U.S. Pat. No. 6,532,824 B1—Ueno et al. disclose a capacitive strain sensor that may be attached to the exterior surface of a hollow cylindrical container for detecting an increase in internal pressure.
U.S. Pat. No. 7,545,119 B1—Egan et al. disclose a piezoelectric sensor mounted on the exterior of the case of an energy storage device, to detect deformation caused by an increase in internal pressure. A voltage caused by deformation of the case wall is compared to a voltage threshold, and the device is shut down if the threshold is breached.
U.S. Pat. No. 7,595,128 B2—Lee et al. disclose a “seesaw member” attached to the outer surface of a battery, whereby swelling causes the seesaw member to exert downward force on a piezoelectric element. The electric current generated by the piezoelectric element is fed to a protection circuit module, which controls the operation of the battery.
U.S. Pat. No. 7,826,189 B1—Edwards discloses a detector for battery swelling. A conductive strip of material having an overlapping portion is attached to the exterior of a battery case. When the case deforms outward, the overlapping portion of the conductive strip decreases, thereby changing the resistance of the strip. The change in resistance is monitored, and the battery may be shut off if failure is determined to be imminent.
U.S. Pat. No. 8,717,186 B2—Zhou discloses a detector for swelling in batteries, incorporating a quantum tunneling composite based pressure-capacitor sensor. The signal generated by the sensor is fed to a microcontroller, and if the signal represents an increase in swelling above a threshold value, the unit is disconnected or replaced.
WO 91/15742—Shelton discloses a pressure sensing capacitor having two planar layers of conductive material separated by a narrow gap. One of the layers is able to flex in response to a change in pressure, thereby narrowing the gap and generating a change in capacitance value.
US Patent Application Publication US 2016/0064780 A1—Jarvis et al. discloses various methods for detecting and controlling battery expansion. The swelling of the battery case may be determined by a capacitive sensor, strain sensor, resistance sensor, acoustic resonance sensor, photo interrupter sensor, contact switch or pressure sensor. A signal indicating that expansion has occurred is sent to a processing unit, and the processing unit modifies the electrical input to the battery.
Despite the various prior art detectors, there remains a need for a device for detecting the expansion of a case housing an energy storage device, while the device continues to function, that is adaptable to a wide range of case designs, inexpensive, robust, and easy to install by an OEM or as an aftermarket add on.